[swift-evolution] [Draft] Hasher & HashVisitable

[David Hart]

> On 14 Mar 2017, at 16:41, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Mar 13, 2017, at 8:38 AM, Vincent Esche via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Source compatibility
>> 
>> Making use of "extending protocols to conform to protocols":
>> 
>> extension Hashable: HashVisitable 
>> {
>> 
>> func hash<H: Hasher>(_ hasher: inout
>> H) {
>> 
>> self.hashValue.hash(&
>> hasher)
>>    }
>> }
> 
> We're unlikely to add this feature soon. It seems reasonable to me to instead have `HashVisitable` refine `Hashable` and provide a default implementation of `hashValue` using a default hasher. I think we still want `Hashable` to be the currency protocol most APIs work with for performance in unspecialized code, since we could inline the visitation and hasher implementation together inside the specialized `hashValue` witness.

Can you explain the performance argument? How does it fare (in your opinion) compared to the arguments in the proposal?

How about:

protocol Hashable {
    func hash<H: Hasher>(with hasher: inout H)
}

extension Hashable {
    var hashValue: Int {
        var hasher = StdLibDefaultHasher()
        hash(with: hasher)
        return hash.finish()
    }
}

> -Joe
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